Numerical study of the influence of tunnel wall properties on ceiling jet temperature in tunnel fires

Abstract In tunnel fires, hot gas, called a ceiling jet, propagates beneath the ceiling from the fire source to the tunnel portals. The properties of the ceiling jet must be understood for conducting a fire safety assessment. Heat loss to the tunnel walls (ceiling and side walls) significantly affects the property of the ceiling jet movement in the tunnel. This study aims to estimate the influence of the tunnel wall property on the gas temperature using numerical simulation. Moreover, the correlation between the gas temperature and thermal parameter of the tunnel walls is derived. The simulation results showed that the gas temperature beneath the ceiling is approximately same when the thermal inertias of the tunnel walls are equal. The influence of heat penetration on the gas temperature was significant, and therefore it was important to confirm thermal penetration while conducting the model experiment. Moreover, it was revealed that a linear relationship exists between the gas temperature rise and a dimensionless parameter, which is a function of the Biot number and the Fourier number. The equation expressing this relationship can be used to estimate the influence of the tunnel wall property on the gas temperature.